A solar cell (or photovoltaic cell) is a core element in solar power generation to directly convert solar light into electricity.
For example, if the solar light having energy greater than bandgap energy of a semiconductor is incident into a solar cell having the PN junction structure, electron-hole pairs are generated. As electrons and holes are collected into an N layer and a P layer, respectively, due to the electric field formed in a PN junction part, photovoltage is generated between the N and P layers. In this case, if a load is connected to electrodes provided at both ends of the solar cell, current flows through the solar cell.
Recently, as energy consumption is increased, solar cells to convert the solar light into electrical energy have been developed.
In particular, a CIGS-based solar cell, which is a PN hetero junction apparatus having a substrate structure including a glass substrate, a metallic back electrode layer, a P-type CIGS-based light absorbing layer, a high-resistance buffer layer, and an N-type window layer, has been extensively used.
Various studies and researches have been performed to improve electrical characteristics of the solar cell, such as low resistance and high transmittance.
Since CdS has a bandgap between those of the P-type CIGS-based light absorbing layer and the N-type window layer and represents superior electrical characteristics, the CdS has been extensively used for the high-resistance buffer layer. However, since the bandgap of the CdS is relatively fixed, the CdS is disadvantageous to form a buffer layer having a wide bandgap.
In addition, since the CdS has the n-type electrical characteristics, techniques for improving the electrical characteristics by forming a P-type buffer layer on the P-type CIGS-based light absorbing layer are requested.